1. Field of the Invention
This invention relates to a method and an apparatus for an anodic treatment on metallic parts. More particularly, the present invention relates to a method and an apparatus for anodizing a surface of the metallic parts.
2. Description of the Related Art
It is known that many metallic components or parts need a final treatment.
Such a surface treatment increases functionality and the lifetime of the part by improving any of various characteristics, such as protection, wear resistance, hardness, electrical conductivity, lubricity or cosmetic value.
One example of such a metallic component is the head of aluminum pistons used in combustion engines. (As used herein an aluminum component is a component at least partially made of aluminum, including aluminum alloys.) The piston head used in the internal combustion engine is placed close to a combustion zone. More particularly this portion of the piston is in contact with hot gases, and therefore, is subject to high-thermal stresses that may cause deformations or changes in the metallurgical structure. This negatively affects the functioning of the piston head.
To reduce this negative effect, a surface of the piston is treated by an anodic treatment in order to develop an anodic oxide coating that protects the metal from the high-thermal stresses. One such apparatus that performs the anodic treatment is disclosed in, for example, Japan Patent Publication (koukai) No. 9-217200 (incorporated herein by reference). According to that publication, as shown in FIG. 7, the apparatus includes a jacket 101, a lid member 102, a mask socket 103, an O-ring 105, an electrolyte bath 106, a nozzle system 107, a cathode 108, and an anode 109. The jacket 101 forms a part of a circulation circuit of electrolyte (reaction medium), and has a substantially cup shape. The jacket 101 has an opening, which is closed by the lid member 102, at its upper end. The electrolyte bath 106 is provided in the jacket 101. A hole in which the mask socket 103 is fitted is formed at the center of the lid member 102. The mask socket 103 is substantially cylindrical in shape, and is provided at its lower opening portion with an inwardly projected flange portion. A piston 104 is placed in the mask socket 103 in an inverted position. Namely, the piston 104 is inserted into the mask socket 103 by the piston head.
The O-ring 105 is placed on flange portion of the mask socket 103. The O-ring 105 contacts a surface of the piston head when the piston 104 is placed in the mask socket 103. This seals a portion of the piston that is not to be anodized. The nozzle system 107, through which the electrolyte is directed to the piston 104, is placed in the electrolyte bath 106. The cathode 108 is provided at an upper portion of the electrolyte bath 106. The anode 109 contacts the piston 104. The apparatus performs the anodic treatment on an end face of the component (piston).
In the anodizing process, the treatment target, i.e., the piston 104, functions as an anode. Hydroxide ions generated by the electrical discharge generate oxygen which is used to oxidize the surface of the piston 104, i.e., the anode, to form the oxide film on the surface of the piston 104. At the same time, however, the interaction of the electrolyte and the cathode 108 generates hydrogen gas, which flows along the current of the electrolyte. This results in hydrogen adhering to the surface of the piston 104. The hydrogen adhered to the piston 104 causes a serious problem that the hydrogen inhibits a stable anodizing reaction of the piston 104.
As mentioned above this problem is especially problematic with this apparatus. Because a flow from the electrolyte bath to the surface of the piston 104 is not separated from the cathode 108, the hydrogen gas generated from the cathode 108 rides the flow to the surface of the piston 104. Namely, the hydrogen adhered to the surface of the piston 104 interferes with the anodizing reaction. As a result, a stable anodic oxide coating is not formed on the surface of the piston 104. The cathode 108 is positioned relative to the piston 104 in order to reduce the loss by the electrical resistance, or improve the productivity. In such case, the closer the interval between the cathode 108 and the piston 104, the higher the tendency that hydrogen adheres to the piston 104.
According to an embodiment of the present invention an improved method for anodizing a component is provided. The method includes providing a container comprising a supply port, a drain port, and a supply passage connecting the supply port and the drain port, at least a portion of the supply passage including a reaction chamber in fluid connection with a surface of the component to be anodized, and supplying an electric current from an electrode positioned fluidly downstream of the component surface. The method further includes supplying a reaction medium from the supply port to the drain port through the supply passage. The reaction medium that is fluidly downstream of the component surface flows toward the drain port without recirculating to the reaction chamber.
In another embodiment, the method may further include at least one seal member separating a first surface of the component to be anodized from a second surface of the component no to be anodized.
According to another aspect of the present invention, an apparatus for anodizing a component is provided. The apparatus includes a container comprising a portion defining a receiving hole for receiving the component into the container, a supply port in the container for supplying a reaction medium, a drain port in the container for draining the reaction medium, a supply passage connecting the supply port and the drain port, at least a portion of the supply passage including a reaction chamber in fluid connection with a surface of the component to be anodized, and an electrode for supplying an electric current, the electrode being positioned fluidly downstream of the component surface. The supply passage causes the reaction medium that is fluidly downstream of the component surface to flow toward the drain port without recirculating to the reaction chamber.
The apparatus may further include a first seal member for separating a first surface of the component to be anodized from a second surface of the component not to be anodized. The apparatus may alternatively include two seal members, wherein the first seal member and a second seal member separate an annular surface portion of the component to be anodized from a remaining surface portion of the component not to be anodized. Preferably, the supply port and the drain port are formed on opposite sides of the container in a radial direction.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.